Mechanical drive machines generally include an engine that is mechanically coupled to one or more traction devices by way of a torque converter and a transmission. As long as the transmission is engaged and the engine is operational, some amount of torque (i.e., creep torque) is transferred via the torque converter and the transmission to the traction devices. In other words, creep torque is torque that is transferred to the traction devices simply by virtue of the machine being in a proper “gear ratio” or propulsion mode (e.g., drive or reverse) and the machine's power source being operational. Thus, even when the accelerator pedal is not depressed, the machine can be propelled by the creep. This creep allows an operator to modulate the speed of the machine with a brake while performing low speed maneuvers.
Electric drive machines are being used in addition to mechanical drive machines both in on-highway and off-highway applications. An electric drive machine consists generally of an engine drivingly coupled to a generator that produces electric power as the engine operates. The electric power is sent to a motor or a series of motors associated with traction devices of the machine to propel the machine.
The motors are generally controlled in response to an operator input. For example, when the operator displaces an acceleration pedal, a driving signal proportional to the displacement is sent to the motors. Thus, when the accelerator pedal is displaced to a maximum position, a maximum current is sent to the motors and the machine is propelled using a maximum torque in a given direction (i.e., forward, reverse). However, when the accelerator pedal is not displaced (i.e., the accelerator pedal is maintained in a neutral position), no current is sent to the motors and, thus, the machine is not propelled. The lack of torque at zero accelerator pedal displacement and slow speeds (i.e., the lack of creep torque) may be problematic in some situations, such as when the machine is stopped on an incline or when the operator desires to perform low speed maneuvers.
One method for providing creep for a hybrid vehicle is disclosed in U.S. Pat. No. 6,590,299 (the '299 patent) by Kuang et al. The '299 patent discloses a vehicle system control (“VSC”). The VSC interprets driver demand (driver demand is determined using PRNDL position, accelerator and brake position, and vehicle speed), then determines when and the amount of creep and hill holding is needed to meet those driver demands while achieving specified vehicle performance (such as fuel economy, emissions and drivability). For example, in a drive-away from stop scenario when the engine is not running, the VSC will request the traction motor to deliver a certain creep torque (to mimic the creep of a conventional vehicle) while the driver is in transition between a braking request and accelerator request (e.g., the transition time between when the driver removes pressure from a brake pedal and applies pressure to an accelerator pedal). The VSC calculates a creep torque based on a predefined function of vehicle speed only used when the accelerator input is zero (not depressed). Alternatively, the strategy can also require no brake being applied (i.e., brake position is zero).
Although the VSC of the '299 patent may provide creep torque to help prevent rollback on a hill, it may still be inefficient and problematic. For example, the VSC of the '299 patent stops applying creep torque when either the accelerator or the brake is depressed. This forces the controller to continually power-up and power-down the motor when the operator taps the brake or the accelerator in a low speed maneuver. Furthermore, the off and on nature of the '299 controller may feel unnatural to operators who are accustomed to the creep provided by a conventional mechanical drive machine.
The disclosed machine system is directed to overcoming one or more of the problems set forth above.